1. Field of the Invention
The present invention relates to a superconducting thin film, a superconducting current path and a superconducting device which utilize the superconducting thin film. More specifically to a superconducting thin film formed of oxide superconductor material, a superconducting current path and a superconducting device utilizing the superconducting thin film.
2. Description of Related Art
A superconducting current path is one of the electronic applications of a superconductor. If all the current paths of a conventional electronic circuit including semiconductor devices is replaced with superconducting current paths, completely, the electronic circuit will operate rapidly with low power consumption. Superconducting signal paths are also expected to reduce the wave form distortion so that the required number of amplifiers can be reduced. Particularly, by using an oxide superconducting material which has been recently advanced in study, it is possible to produce a superconducting current path through which superconducting current flows at relatively high temperature.
An oxide superconductor has the largest critical current density J.sub.c in direction perpendicular to c-axes of its crystal lattices. Therefore, it is desirable that the superconducting current path through which superconducting current flows horizontally is formed of a c-axis orientated oxide superconductor thin film and the superconducting path through which superconducting current flows vertically is formed of oxide superconductor thin films of which c-axes are orientated horizontally. In this specification, this oxide superconductor thin film of which c-axes are orientated horizontally will be called an "a-axis orientated oxide superconductor thin film".
On the other hand, devices which utilize superconducting phenomena operate rapidly with low power consumption so that they have higher performance than conventional semiconductor devices. Like the superconducting current path, by using an oxide superconducting material, it is possible to produce a superconducting device which operates at relatively high temperature.
Josephson device is one of well-known superconducting device. However, since a Josephson device is a two-terminal device, a logic gate which utilizes Josephson devices becomes complicated. Therefore, three-terminal superconducting devices are more practical.
Typical three-terminal superconducting devices include two types of super-FET (field effect transistor). A first type of the super-FET includes a semiconductor channel, and a superconductor source electrode and a superconductor drain electrode which are formed closely to each other on both sides of the semiconductor channel. A portion of the semiconductor layer between the superconductor source electrode and the superconductor drain electrode has a greatly recessed or undercut rear surface so as to have a reduced thickness. In addition, a gate electrode is formed through a gate insulator layer on the portion of the recessed or undercut rear surface of the semiconductor layer between the superconductor source electrode and the superconductor drain electrode.
A superconducting current flows through the semiconductor layer (channel) between the superconductor source electrode and the superconductor drain electrode due to a superconducting proximity effect, and is controlled by an applied gate voltage. This type of the super-FET operates at a higher speed with a low power consumption.
A second type of the super-FET includes a channel of a superconductor formed between a source electrode and a drain electrode on a substrate, so that a current flowing through the superconducting channel is controlled by a voltage applied to a gate formed above the superconducting channel.
Both of the super-FETs mentioned above are voltage controlled devices which are capable of isolating the output signal from the input signal and of having a well defined gain.
However, since the first type of the super-FET utilizes the superconducting proximity effect, the superconductor source electrode and the superconductor drain electrode have to be positioned within a distance of a few times the coherence length of the superconductor materials of the superconductor source electrode and the superconductor drain electrode. In particular, since an oxide superconductor has a short coherence length, a distance between the superconductor source electrode and the superconductor drain electrode has to be made less than about a few tens of nanometers, if the superconductor source electrode and the superconductor drain electrode are formed of the oxide superconductor material. However, it is very difficult to conduct a fine processing such as a fine pattern etching, so as to satisfy the very short channel distance mentioned above.
On the other hand, the super-FET having the superconducting channel has a large current capability, and the fine processing which is required to produce the first type of the super-FET is not needed to product this type of super-FET.
In order to obtain a complete ON/OFF operation, both of the superconducting channel and the gate insulating layer should have an extremely thin thickness. For example, the superconducting channel formed of an oxide superconductor material should have a thickness of less than five nanometers, and the gate insulating layer should have a thickness more than ten nanometers which is sufficient to prevent a tunnel current.
This extremely thin superconducting channel is insufficient for thickness as a source region and a drain region from and to which the superconducting current flows through the superconducting channel. Therefore, in the super-FET, a superconducting source region and a superconducting drain region having a relatively thick thickness should be arranged at the both side of the superconducting channel.
In this connection, the main current flows in parallel to the substrate through the superconducting channel and flows perpendicular to the substrate in the superconducting source region and superconducting drain region.
An oxide superconductor has the largest critical current density J.sub.c in direction perpendicular to c-axes of its crystal lattices. Therefore, it is desirable that the superconducting channel is formed of a c-axis orientated oxide superconductor thin film and the superconducting source region and the superconducting drain region are formed of a-axis orientated oxide superconductor thin films.
However, grain boundaries are generated at the interface between the c-axis orientated oxide superconductor thin film and the a-axis orientated oxide superconductor thin film, which introduce difficulties of superconducting current flowing. The grain boundaries sometimes form Josephson junctions which pass only tunnel current so that the current capability is limited and the input and output characteristics become nonlinear. If no Josephson junction is formed at the interface, Joule heat may be generated by the electric resistance formed at the interface, which causes the "quench" phenomenon. The c-axis orientated oxide superconductor thin film and the a-axis orientated oxide superconductor thin film may interfere with each other so as to degrade each other.
In the prior art, there has been proposed that a metal layer of Au, Ag, etc. is inserted between the c-axis orientated oxide superconductor thin film and the a-axis orientated oxide superconductor thin film so that the interface does not consist of the grain boundaries of the oxide superconductor. However, even if the metal layer is formed of Au or Ag, it still sustains electric resistance so that the "quench" may occur.